One of the major problems inhibiting widespread applications of oxide superconductors particularly to either large-scale or complicated systems, for example, for superconducting transmission cables, bus bar, nuclear magnetic resonance (NMR) systems, medical diagnostic tools (e.g. magnetic resonance imaging (MRI)), superconducting magnet energy storage systems (SMES), superconducting generators single-crystal drawing-up systems in a magnetic field, freezer cooling superconducting magnets, nuclear fission reactor magnets, accelerators, magnetic separation units, large scale devices, current leads and the like, where it is necessary not only to pass transport current as high as 103 A or more but is also essential to have a long conductor with stable superconductivity along its entire length. And it is impossible to fabricate the whole system in a single piece. Consequently, it is frequently necessary to join superconductors to each other (end-to-end) in order to form a joined superconductor of sufficient length, which is stabilized and is in a state capable of ensuring a heavy current. Thus it is very essential that these joints between such superconductors must have the same superconducting properties, as of the original shorter length itself to be useful in a given application.
Further, it is also necessary that a technique be available for removing and replacing a section of the superconducting material if it becomes damaged by the thermal/mechanical shocks while working. The contact between the replaced new piece and the undamaged sections of the original piece must be superconducting and also should have the same current capacity as the original and replaced piece themselves.
So far established technique for making superconducting joint for metal superconductors, such as NbTi and Nb3Sn is in the wire form. In contrast to these metal superconductors, the situation for oxide superconductors is particularly problematic due to their brittle nature, poor ductility and low breaking resistance etc.
To produce joints between oxide superconductors, similar methods like overlapping fusing, laser welding, spot welding, soldering, brazing, lap joining, butt joining, ultrasonic joining, laminate joining etc. as used for metal superconductors are developed. However, these are related to Ag-clad wires/tapes of both mono and multifilaments [Japan. J. Appl. Physics, vol. 34, p. 4770 (1995); U.S. Pat. Nos. 6,133,814; 6,159,905, 6,753,748, Appl. Supercond, vol, 3, p 207 (1995); Supercond. Sci. Technol. Vol, 13, p. 237 (2000); U.S. Pat. No. 6,194,2226]. On the other hand, very few methods for joining oxide bulk superconductors in bulk form like bars, disks, rods etc. have been proposed up to now. But these are now beginning to attract more attention.
Reference may be made to two Japanese Patents Laid-open No. Hei 1 (1989)-24379; and Japanese Patent Laid-open No. Hei 1 (1989)-17384, wherein, the joint parts are produced through solid state reaction. Both these methods lead to randomly oriented and low-density joined parts that a sufficient critical current density (Jc) is hard to procure.
In another reference: “Journal of the Electrochemical Society, volume 136, Number 2, pp 582-583 (February 1989)”, Y. Tzeng discloses an improved method, wherein the joint portion is produced through melting using an autogeneous welding technique in which end faces of two bar pieces of preformed superconducting bulk YBa2 Cu3O7-x are overlapped, heated to melt and fused together with a subsequent recovery heating of the joined portion. It is taught that the joint having Jc which is about the same as that of the original component bars can be produced by this method.
In U.S. Pat. No. 5,244,876, a similar autogeneous welding technique to join preformed silver sheathed Bi2Sr2Ca1Cu2O8+x superconducting rods/disks, has been used but with a change. Instead of overlapping, the disclosure places end faces of the two superconductors close together with a gap and filling this gap with molten material of the same superconducting rod followed by recovery heating of the joined portion.
The above two known joining methods have the following problems. The joint is generally resistive, because the joint parts thereof are produced through melting of the preformed superconducting phase, that a full recovery of damaged superconducting phase is hard to procure.
Li et. al. in reference: J. Less-Common Met. Vol-164/165, p. 660 (1990), disclosed a welding method for joining of preformed Bi-based superconducting bars by using an LPG-O2 flame. It is taught that during welding, the top of the samples soon melted and a gap appears, and the tiny particles from the same melt is added to fill the gap. Then reporting formation of a superconducting joint after a heat treatment. However, drawback of this method is that the joint portion is larger, which makes the joint mechanically poor.
J. Cai et. al in reference: Supercond. Sci. Technol. volume 5, p 599 (1992) disclosed a Microwave technique for joining of (Bi,Pb)-2223 preformed superconducting bars end to end under an axial compressive load and then reporting mechanically strong joint. However, drawback of this method is that the disclosures do not disclose about the superconducting properties of the joint.
In U.S. Pat. Nos. 5,116,810 and 5,321,003, both to Joshi et al, also make use of a similar method but in contrast to joint formation between preformed superconducting components, the joint according to this patent is formed between shaped pieces of metallic precursor rare earth elements and the joined portion is spot welded with a subsequent heating to form superconducting phase. Although, the joints formed prior to formation of superconducting phase are substantially non-resistive in nature, however, it is frequently inconvenient to perform such joining operations on elemental precursors, rather than on components in the superconducting state.
Mutoh et. al. in Japanese J. Apply. Phys. Vol. 29, No. 8, p L1432 (August 1990) disclosed a method using (Bi,Pb)-2223 tablets pumped up from melts. Joint is formed between a pair of such tablets: unannealed, annealed and annealed with an unannealed insert plate. Two tablets aligned, one on top of the other, and hot pressed at various temperatures and pressures. Then reporting, a good superconducting joint can only be obtained when annealed starting melt (i.e. second case) was used. Drawback of this method lies in the presence of unavoidable traces of impurity phases both in the bulk as well as in the joined portion, thereby leading to a poor quality end product.
In another reference: Jap. J. Appl. Phys. Vol. 29, p L875 (1990), although the disclosure used the same hot pressing method as used in the above citation, for joining of thick films, however, concludes differently. That is, according to this study, acceptable joints are only possible with completely reacted material rather than precursor material for the hot pressing process. However, drawback of this method is that weak superconducting joints are formed.
In the methods described in Japan Patents Laid-open Nos. 5 Hei 3 (1991)-242384; Hei 3 (1991)-254473, use is made of partial melt solidification (i.e. crystallization from the melted state) of the preformed superconducting bodies of Bi-2212 to be joined to produce a dense and oriented matrix in the joint portion so as to result in a superconducting joint. For this method of crystallization from the melted state, use of a method of insertion of an intervening substance having lower melting point than that of the connecting bodies is made. According to the method in former patent an intervening substance is either a precursor or calcined body in powder form which is filled in the joint part, followed by thermal treatment for crystallization from the melted state. Whereas in this method, the joint part is largely oriented and dense in comparison to the above methods. This method, however, has a draw back due to difference in thermal treatments (by about 10° C.) of the bodies to be joined and of the intervening substance because of their different melting temperatures, good jointing can be attained only with much difficulty. Further, if the intervening substance is singly melted, the crystal growing in liquid phase is hardly connected to the bodies to be joined.
In the method disclosed in the latter patent, the intervening substance is a calcined powder containing Bi-2212 phase. Disadvantage of this method lies in the reduction of crystal orientation at the joint part as compared with the orientation within the component parts due to variation in the melting temperature as a result of changing ratio of the calcined powder mixed to the Bi-2212 phase as the heating proceeds. Consequently, it leads to reduction of Ic at the joint part.
An improvement in the above method is disclosed in U.S. Pat. No. 6,258,754, wherein the overall assembly of joining YbBa2Cu3O7−Y powder sandwiched between two independent performed superconducting single YBa2Cu3O7−x domains and of the joint portion is heat-treated such that melting occurs only in the interface and the bonding material grows epitaxially. Then reporting strongly linked bars. Rings (rectangular or square) can also be joined together by stacking one over the other. This method however, has a drawback when a powder is used as the joining material, the air present between the particles cannot escape even in the course of melting and solidification of the joining material and therefore, result in not only pores but also segregation of impurities in the joined portion.
To overcome the problem occurring due to trapped air with the use of powder of YbBa2Cu3O7−Y as the joining material in the above patents, Iida et. al. in U.S. Pat. No. 7,001,870 made use of a sintered dense material of YbBa2Cu3O7−Y which is interposed between two sintered YbBa2Cu3O7−Y blocks, and this sintered joining material is melted and then solidified to form a joining layer, thereby joining the YbBa2Cu3O7−x rectangular blocks. Although, this sintered material provided a relatively better joint, however, the drawback of this method is that, even a slight difference in the recrystallization points, crystal structures etc. of the joining material from that of the YbBa2Cu3O7−x blocks fails to make a strong superconducting joint.
All the above methods having their own merits and limitations are related to joining of bulk bodies in solid form: like wires/tapes, Ag-sheathed rods, ribbons, sheets, bars, blocks and in hollow form: like rings having rectangular or square cross section and not having circular cross-section in order to keep the interface angle below 10 degree for easy joining and that too by stacking samples of small size in millimeter range. Thus, attention has not been paid to a method for joining together hollow bodies of large size and having circular cross section, like tube conductors which are now under rapid technical development due to their Ic values which are not much limited by the self-magnetic field in comparison to those of the solid rod conductors.